1. Field of the Invention
The present invention relates to a leadframe for mounting a semiconductor chip, and to a method of manufacturing a semiconductor device by use of such a leadframe.
2. Description of the Prior Art
A semiconductor chip has various devices and circuits, such as transistors, diodes, and integrated circuits (ICs), formed in it. Usually, a semiconductor chip is, in the last steps of its manufacture, fitted with as many leads as required and resin-molded for protection, so that it is at last made into a semiconductor device as an end product. In these steps, a leadframe is used. A leadframe is made of metal, and consists of a portion that is later made into individual leads and a portion on which a semiconductor chip is mounted. The portion for mounting the semiconductor chip is called the "island", and is formed at the tip-side end of one lead. The tip-side ends of the other leads are so formed as to come in the vicinity of the island.
On the top surface of the semiconductor chip, a plurality of bonding pads for input or output are provided that are connected to the circuits formed inside. The semiconductor chip is fixed on the island with adhesive, and is then connected to the tip-side ends of the leads by wire-bonding. Depending on the circuits formed inside, the semiconductor chip may be, for example, so constructed that terminals such as input, output, and power-supply terminals are drawn out from its bottom surface. A semiconductor chip of this type is fixed on the island with conductive adhesive and is at the same time electrically connected to one lead. In a semiconductor chip of the type that does not have terminals drawn out from its bottom surface, one bonding pad is in some cases connected to the peripheral part of the island with a wire.
A resin mold is formed so as to enclose the semiconductor chip, the island, and the tip-side ends of the leads. The portions of the leads that are enclosed in the mold are called the "inner leads", and their portions that are left outside the mold are called the "outer leads".
To enhance productivity, it is preferable, instead of performing the above steps (i.e. fixing the semiconductor chip on the island, connecting the semiconductor chip to the leads, and forming the resin mold) separately on individual semiconductor chips, to perform each step on a plurality of semiconductor chips successively or simultaneously. To achieve this, a leadframe usually has many sets of an island and leads arrayed at regular intervals. In such a leadframe, all the leads are joined together not only at their base-side ends, but also at intermediate positions between their base-side and tip-side ends. This is to prevent warping of the leadframe and variations in the lead-to-lead intervals. The portion that joins the leads together at their intermediate positions is called the "tie-bar".
FIG. 5 shows a conventional leadframe after the formation of resin molds. In FIG. 5, numeral 27 represents a resin mold, numerals 21a and 21b in combination represent an outer lead 21 of a lead 20, and numeral 23 represents a tie-bar. Note that no inner leads appear in the figure because they are enclosed in the resin molds. Note also that, in this figure, only part of the leadframe is shown, that is, its portions extending rightward and leftward as well as the base-side ends of the leads are omitted. In this example, one semiconductor device is given three leads 20.
The individual semiconductor devices are separated from one another by cutting apart the tie-bar 23 joining the leads 20 and cutting apart the outer leads 21 at their base-side ends. The outer leads 21 of a separated semiconductor device are, if necessary, bent so that they run along the external surface of the resin mold 27. A semiconductor device as an end product is mounted on a circuit board together with other components. In that case, the semiconductor device is firmly mounted on the circuit board by soldering the outer leads 21 of the former to the wiring pattern formed on the latter.
It is desirable that semiconductor chips and semiconductor devices be made as small as possible. Accordingly, the pitch between the inner leads (indicated by the letter A in FIG. 5) is made as small as possible. On the other hand, since mounting is achieved by use of solder, to prevent short circuits, the pitch between the outer leads 21 in their portions 21b farther from the island (indicated by the letter B in FIG. 5) needs to be made comparatively large. For this reason, not all the leads 20 are made linear, but at least those situated outside are so formed as to have bends.
A leadframe is formed by punching it out of a metal sheet into a desired shape. Conventionally, as shown in FIG. 5, to simplify the shape to be punched out, parts of the tie-bar 23 joining the leads 20 are used also as the bends 21c of the leads 20. As a result, the outer leads 21 have different pitches on the two sides of the tie-bar 23; that is, they have, in their portions 21a, a pitch equal to the pitch A of the inner leads and, in their portions 21b, a larger pitch B. FIG. 6 shows an enlarged view of the lead 20 after the tie-bar 23 has been cut apart. As shown there, only those parts of the tie-bar 23 which join adjacent leads are cut away, and the other parts are left behind as the bends 21c of the two outside leads.
For a semiconductor chip that requires as few as three leads, bends in its leads do not bring the larger-pitched portion 21b of one lead too close to the smaller-pitched portion 21a of the adjacent lead. Therefore, it is possible to cut apart the tie-bar 23 parallel to the side edges of the leads 20 and still secure sufficiently wide gaps between the leads there.
However, for a semiconductor chip that requires more leads, bends in its leads bring the larger-pitched portion of one lead too close to the smaller-pitched portion of the adjacent lead. FIG. 7 shows a leadframe that requires four leads, and FIG. 8 shows an enlarged view of its lead 20 after the tie-bar 23 has been cut apart. In this case, not only the two outside leads, but also the two inside leads have bends 21c, and the portions 21b of the two inside leads are brought close to the portions 21a of the two outside leads. Therefore, to secure as wide gaps as possible between the leads after the tie-bar 23 has been cut apart, it is necessary, as shown in FIG. 8, to cut apart the tie-bar 23 not parallel to the side edges of the leads 20 but obliquely thereto.
However, even if the tie-bar is cut apart obliquely, there is a limit to widening the gap between the leads there. In addition, oblique cutting requires very accurate positioning, and, unless sufficient positioning accuracy is secured, the gaps between the leads will no longer be uniform, that is, some gaps will be left smaller than desired. When a semiconductor device is mounted on a circuit board, gaps that are smaller than desired tend to readily cause adjacent leads to be brought into contact and thus short-circuited by solder, especially when solder is applied so profusely as to form swells.
To prevent short circuits between the leads, non-conductive resin is often applied to the surfaces of the leads at those positions where the tie-bar is cut apart, thereby to form non-conductive coatings. In the example shown in FIG. 8, non-conductive coatings are formed in the area enclosed by broken lines.
Forming non-conductive coatings at the positions where the tie-bar is cut apart is useful in securing insulation between the leads. However, the non-conductive coatings need to be formed after the tie-bar has been cut apart, that is, separately for individual semiconductor devices. This not only increases manufacturing steps but also greatly diminishes the productivity of semiconductor devices. In addition, in semiconductor devices such as light-emitting diodes, light-receiving ICs, and photointerruptors that employ photoelectric conversion devices, there is a possibility that the openings provided to let light in or out are erroneously filled up with the resin applied as non-conductive coatings when too much resin is applied.
Furthermore, to secure sufficient positioning accuracy in cutting the tie-bar obliquely, the leadframe needs to be produced with higher accuracy. Thus, for reasons as described hitherto, the structure of conventional leadframes has been curbing reduction of the manufacturing cost of semiconductor devices as well as improvement of their yield.
The fixing of the semiconductor chip on the island and the wire-bonding thereof are performed, with the leadframe placed on a flat stage, by pressing down the tie-bar-side portions of the inner leads from above with a pin. FIG. 9 shows the side view of the island during wire-bonding. In FIG. 9, numeral 31 represents an island, numeral 32 represents an inner lead, numeral 33 represents a semiconductor chip, numeral 34 represents a bonding pad, numeral 35 represents a wire, numeral 36 represents a stage, numeral 37 represents a capillary tool, and numeral 38 represents a pressing pin. Usually, the island 31 and the lead 32 are both placed in close contact with the top surface of the stage 36, and the semiconductor chip 33 is placed in close contact with the top surface of the island 31.
From this state, the capillary tool 37 is moved down so that the wire 35 protruding from its tip is pressed against the bonding pad 34 with force that is weak enough not to damage the semiconductor chip 33. Then, from the capillary tool 37, a supersonic wave is applied to the tip of the wire 35 and thereby the wire 35 is fixed to the bonding bad 34. Subsequently, the capillary tool 37 is moved to above the lead 32 and then down onto it so that the wire 35 is fixed to the lead 32 and then cut apart with force and a supersonic wave in a similar way.
However, since the island is farther than the tip of any lead from the tie-bar, the tip of the island tends to bend upward and come off the upper surface of the stage. In FIG. 9, the dash-and-dot lines show the state of the island 31 when its tip has come off the stage 36. In this state, it is impossible to fix the semiconductor chip 33 on the island 31 so that the former is placed in close contact with the latter, and thus it is impossible to obtain sufficient fitting strength.
In addition, if wire-bonding is performed with the tip of the island 31 off the stage 36, the island 31 bends freely during wire-bonding and thereby not only prevents the application of sufficient force to the wire 35, but also scatters around the supersonic wave. As a result, it is impossible to connect the wire 35 to the bonding pad 34 properly. Similarly, when the wire 35 is connected to the peripheral part of the island 31, it is impossible to secure proper connection between them.
To prevent such coming-off of the island 31, when the semiconductor chip is fixed on the island and is wire-bonded, it is customary to press down the tip of the island from above with another pin. In FIG. 9, numeral 39 represents such a pin that is used to press the tip of the island 31 against the stage 36. However, to accommodate this pin, the island 31 needs to have an area larger than is necessary to mount the semiconductor chip, and therefore the resin mold needs to be made accordingly larger. This inevitably makes the semiconductor device as an end product unduly large.